Favorite: An innocent and inexperienced (usually virgin) but not completely naive young man is kidnapped by one or two more dominant characters to be raped and used as a sex slave. Occasionally interested in seeing more get in the mix, but only one or two main doms.
Chained/tied up, moderate whipping/spanking, toys (but not very fond of small vibes or anal beads or chastity restraints for the dick), degrading language, light/moderate cuts/bruising/injuries, use of collar/leash if the mood is right. Like victims to resist/try to fight back a little, but ultimately too weak and are subdued, but still don't willingly give in or get mind controlled or anything like that. Doms/rapists are tough and 100% in control.
If the mood is right: Multiple slaves/mass orgy/gangrape, buying/selling, blackmail, blindfolds, shibari, latex, tables turned on dom/rapist (say another character gets revenge on him for the victim) but usually only at the end of the story.
I also like voluntary sub/Dom stories, but usually with softer stuff, not what would count as actual slavery.
Complete turn-offs: Genital piercings/weights/abuse, pet play (acting like a dog/animal, pony play/tail butt plugs), heavy and excessive abuse (blood/gore/dismemberment) especially with no sexual context, scat/piss, murder/snuff, victim falling in love with rapist or developing stockholm syndrome.